Fig. 1. (a) Schematic of the optical contrast definition. (b) A color plot of the contrast as a function of the wavelength and the SiO2 thickness^[14].

Fig. 2. (a) Contrast spectra of graphene with different LNs on 300 nm SiO2 substrate. (b) The optical images of all the samples in (a). The graphene flakes in a, b, c, d, e, and f are more than 10 layers and the thickness increases from a to f^[19].

Fig. 3. The G and 2D modes of graphene vary with different layers^[27].

Fig. 4. (a) Four active Raman modes in bulk MoS2. (b) Raman spectra of atomically thin and bulk MoS2 films. (c) The frequency shifts of two characteristic Raman modes and their difference as a function of LNs^[29].

Fig. 5. (a) Raman spectra of BP with different numbers of layers. (b) The schematic structural view of monolayer BP showing the crystal orientation. (c) The polarization-resolved Raman scattering spectra of monolayer BP with linearly polarized laser excitation. (d) The intensity of the Ag1 mode as a function of the laser polarization angle in the x-y plane^[33].

Fig. 6. Calculated band structure of (a) bulk, (b) quadrilayer, (c) bilayer, and (d) monolayer MoS2^[37].

Fig. 7. (a) PL spectra of monolayer and bilayer MoS2. (b) The normalized PL spectra of MoS2 with increasing LNs (c) The evolution of the bandgap with increasing MoS2 LNs^[29].

Fig. 8. SHG of (a) MoS2 and (b) h-BN with different layers^[43].

Fig. 9. (a) Optical micrograph of MoS2. (b) The SHG and (c) the THG images with few-layer areas under 1560 nm excitation. (d) The intensity of the SHG and THG as a function of LNs^[47].